22
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
Proceedings of the 18
th
International Conference on Soil Mechanics and Geotechnical Engineering, Paris 2013
and 18º, and perhaps 20º in sections close to the toe. The height
of the slope involved in the sliding was about 22 m. The slide
area had a width of 275 m parallel to the watercourse and a
length of 150 m normal to the watercourse. A total area of
54,000 m
2
was affected by the landslide.
The morphology of the slide was typical of a spread (Varnes,
1078). The debris were a succession of long slices of deformed
and dislocated material oriented normal to the direction of
movement. Some of the debris took the form of a triangular
prism and reminded of horsts, and these were displaced horizon-
tally only. The horsts were separated by slices of relatively
undisturbed material, just like a block having dropped due to
some sort of faulting, and were called grabens (Fig. 18). In the
back part of the slide, some blocks coming from an upper
failure surface were pushed upward by movement and
overlapped the adjacent lower slices.
The investigation of the landslide was carried out by the
Geotechnical and Geological Department of the Ministry of
Transportation of Québec (Locat
et al
2011).
Figure 17. Saint-Jude landslide and location of Salvail river (dashed
line) (Locat
et al
2011).
Figure 18. Saint-Jude landslide: illustration of the horsts and grabens af-
ter the slide (Locat
et al
2011).
6.2
Soil parameters
The soils involved in the landslide are mainly marine clay from
the former Champlain Sea. The clay was sensitive, of medium
to firm consistency, and had sensitivity ranging from 30 to 80
and liquidity index decreasing form 2 to 1 with depth. There
was artesian pressure of 10 m above the river level at the land-
slide site. Table 7 and 8 describe a typical soil profile.
Figure 19 gives an example of the cone resistance measured
at the site after the failure. The profiling enabled the determina-
tion of the location of the slip surface (Elevation +4 in Fig. 19).
6.3
Analysis of the landslide
The analysis of the available data revealed that landslides had
occurred earlier along the Salvail River. Aerial photographs
since 1950 showed that erosion was active more or less steadily
at the foot of the slope and that small landslides associated with
erosion had occurred. The erosion seemed to have intensified
over the past 15 years.
Table 7. Index properties, Saint-Jude slide (after Locat
et al
2011).
Depth
(m)
Soil description
w
(%)
I
p
(%)
I
L
0-3.8
Drying crust, sandy,
silty from 2 m
24-78
-
-
3.8-26
Clay, some silt traces
of sand
65
20-37
2.0-1.0
26-31
Silty clay
45-75
21-37
0.7-1.0
31-37
Silt, clayey, some
sand, trace gravel
15-25
12-29
0.5-1.5
37-42.6
Silt, sandy, some clay,
trace gravel, v.dense
13-18
-
-
>42.6
Shale and sandstone
-
-
-
w
water content
I
p
plasticity index
I
L
liquidity index
Table 8. Stress and strength characteristics, Saint-Jude slide location
(after Locat
et al
2011).
Depth
(m)
Soil description
s
u
(kPa)
p'
c
(kPa)
(kN/m
3
)
0-3.8
Drying crust, sandy,
silty from 2 m
50-165
250-400
18.6
3.8-26
Clay, some silt
traces of sand
25-65
100-260
16.0
26-31
Clay, sandy
50-107
180-310
16.8
31-37
Silt, clayey, some
sand, trace gravel
40-150
-
19.3
37-42.6
Silt, sandy, some
clay, trace gravel
-
-
20.7
>42.6
Shale and sandstone
-
-
-
s
u
undrained shear strength from field vane (CPTU N
kt
= 13.5)
p'
c
preconsolidation stress (OCR = 1.4 at El. +15 and 1.0 at El. 0)
soil unit weight
Figure 19. Example of piezocone results in Saint-Jude deposit and indi-
cation of slip surface at Elevation +4 (Locat
et al
2011).
The slide is believed to have occurred as follows. Along the
centreline of the slide, the slip surface developed at depth and
